The movie opens with Cody Jarrett (James Cagney) and his gang robbing a train for over $300,000. Two train engineers are killed during the robbery. The gang holes up in a cabin along with Cody's wife (Virginia Mayo) and his devoted mother, who are just as crooked as Cody. Although he's a strong leader, Cody suffers from powerful headaches, and his gang often gets impatient because he prefers to wait a long time between robberies to avoid capture. One of his cronies, Big Ed, is especially eager to get Cody out of the picture and take control of the gang, but he knows it's not his time - although that doesn't stop him from making eyes with Cody's wife, unbeknownst to Cody. Finally, a harsh storm moves into the area, allowing Cody, his family, and his gang to escape to another state

From forensic evidence, the Feds, led by Treasury Agent Evans, are able to piece together the involvement of the Jarrett gang in the train robbery. On a tip from an agent in the field, Evans learns the whereabouts of Ma Jarrett. The field agent puts a marker on Ma Jarrett's car as she's shopping for strawberries, and Evans tries to tail Ma in the hopes that she'll lead him back to Cody and the gang. Ma Jarrett's no fool, and she nearly loses Evans, but he eventually does track her down to Cody's location and covers the hideout alone while his partner calls for backup. Cody, trusting Ma's hunch that she's been followed - Cody loves, trusts, and depends upon his Ma more than anybody else in the whole world - decides it's time to leave. Evans confronts Cody as Cody tries to prepare the getaway car, but Cody wounds Evans with a hidden pistol, and Cody and his wife and Ma manage to elude capture by parking at a drive-in movie. As the movies, Cody explains that to avoid the gas chamber for killing those engineers during the train robbery he will confess to a lesser crime performed about the same time as the robbery. Ma Jarrett agrees to ride herd on the gang while Cody's locked up, and his wife agrees to wait for him until he gets out - though her sincerety is very much in question.

When the Feds hear that Cody has copped to a lesser crime in another state, they decide to let Cody think he's tricked everybody, but in reality they plant an undercover agent named Hank Fallon (Edmund O' Brien) in jail with Cody in the hopes of discovering who gives Cody all the great plans and schemes by which Cody pulls off his heists, and also how Cody manages to launder all the money he steals. Fallon earns Cody's trust by saving Cody from being "accidentally" killed by one of Big Ed's friends on the inside, and also by protecting Cody during one of Cody's blinding headaches. When Ma Jarrett hears of the attempt on her son's life, she tells Cody during visiting hours, much to his horror, that she plans to kill Big Ed. Cody is now desperate to get out of jail so that he can protect his Ma, and Fallon comes up with a safe plan to do it - just him and Cody would escape. However, on the day of the planned prison break, Cody learns from another inmate that Ma Jarrett is dead, and he totally flips out, punching out guards and getting sent to solitary in a straitjacket for his trouble. But Cody's still on the ball, and he concocts his own plan which successfully breaks himself and his whole gang - now including Fallon - out of prison.

Cody's first stop is his old hideout, where he kills Big Ed and reclaims his wife (who, unbeknownst to Cody, is the one who shot his Ma in the back). His next stop is a house where his gang on the outside has stashed a gas truck they bought with the proceeds of their last robbery. At the house Cody meets up with "the Trader", who is the very person whose identity Fallon was planted to discover in the first place. After learning "the Trader's" real name, the means of his money-laundering, and the plans for Cody's next heist, Fallon tries to get word out to Evans but can't because Cody is watching him. He does manage, however, to leave a message for Evans in a gas station washroom and set up a transmitter that will tell Evans the gang's position. The gas station attendant sees the message Fallon left and gets word to Evans just in time, for Cody's next heist is already underway. This time Cody plans to rob the payroll of a gas refinery, but Evans, having followed the transmitter's signal, shows up and surrounds the place with federal agents, trapping Cody and his men inside - except for Fallon, who escapes and rejoins his federal collegues. Cody's men are picked off by cops and snipers one by one until only Cody is left, and after taking three bullets himself, Cody intentionally starts shooting at gas pipes, starting a fire that eventually will lead to a massive explosion. The cops flee to safety, and right before the explosion, Cody shouts his famous final words, "MADE IT, MA!!! TOP OF THE WORLD!!!"

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